Electroreprographic systems, and xerographic systems in particular, use corona producing devices to produce electric fields to, for example, charge retentive photoresponsive surfaces, such as photoreceptor belt or drum surfaces. Various types of such corona charge generating devices include wires, while others include pins or teeth. In all cases, charge uniformity is desirable, and various solutions have been presented to make the fields produced by corona charge generating devices more uniform. U.S. Pat. Nos. 5,324,942; 2,777,957; 2,965,754; 3,937,960; 4,112,299; 4,456,365; 4,638,397; and 5,025,155 disclose various prior art corona charge producing devices; the disclosures of these patents are incorporated by reference into the disclosure of the instant patent application. Xerox Disclosure Journal (Vol. 10, No. 3; May/June 1985) teaches, at pp. 139–140, an alternate approach; the disclosure of this article is also incorporated by reference into the instant patent application.
FIG. 3 shows a typical prior art saw tooth corona producing array in which all teeth project the same amount toward the photoreceptor. Such a uniform amount of tooth projection yields a non-uniform charging potential profile, as seen in FIG. 4, with teeth toward the center of the array having a decreasing contribution. As illustrated by these FIGS. and by the disclosures of the references mentioned above, current design of saw tooth and pin array based corona producing devices are prone to non-uniform charging patterns. Referring to the pins and teeth of such devices as elements, we see that this variation in charging pattern is due to a fundamental problem that causes the electric field to be highest at the edge elements. This is due in part to shielding effects evinced by adjacent elements, so that as one examines the field produced by elements toward the center of an array, one sees lower values since the field from other elements is blocked by the presence of intervening elements. The corona supply therefore is highest near the edge of the charging device. If the print area near the edges is not carefully selected, a dark edge may result in the print.
This effect can be understood from the symmetry and shielding of electric field by neighboring elements. The elements that lie inside the array have symmetrical flow of corona current on both sides, but the elements that lie near the edges have corona current only on one side of the pins. The electric field at the heads of inside elements, therefore, is reduced. As the voltage applied to the array is raised, the outside elements begin to glow first because the threshold field for air breakdown is reached there first. With further rise of voltage, other elements also glow, but the respective current is lower. This can be seen in the lower intensity of glow at these elements. The voltage profile deposited by a corotron or scorotron with such a uniform element projection profile has peaks under the outside edges.
To overcome such non-uniform voltage profiles, embodiments provide a charging apparatus that applies a substantially uniform charge to a charge retentive surface. The apparatus comprises a corona producing device, spaced from the charge retentive surface, that emits corona ions, but with corona producing elements of varying heights. The height of the elements near the edges is reduced so that the distance between the surface to be charged and the ends of the edge elements is greater than that between the surface to be charged and the ends of the inner elements. The actual height is found, for example, by iterative calculation as will be shown below.